[21.05] Fe XXV and Fe XXVI Diagnostics of the Black Hole and Accretion Disk in AGN

We report the detection of Fe~{\sc xxv} and Fe~{\sc xxvi}
K\alpha emission lines from a Chandra HETGS
observation of the narrow-line Seyfert~1 galaxy NGC~7314.
The lines are redshifted (cz ~1500 \rm \ km \ s-1)
and unresolved. The most viable geometry for the origin of
the lines is a disk. The redshift and width constraints on
Fe~{\sc xxvi}~Ly\alpha alone imply that the disk is within
7\circ of face-on, and that the line emission comes
from within ~600 gravitational radii of the putative
central black hole. These limits are model-independent: they
not depend on the radial line emissivity, nor on how the
continuum is modeled. The only requirement is that the
Fe~{\sc xxvi}~Ly\alpha line indeed originates in a flat,
Keplerian disk. The line emission may extend all the way
down to a few gravitational radii and yet still give an
overall line profile which is unresolved by Chandra if
the disk is illuminated approximately uniformly (this is
true also for the more common 6.4~keV K\alpha lines). The
ionization balance of Fe responds to rapid continuum
variations on timescales less than 12.5~ks -- unresolved
emission from ionization states lower than Fe~{\sc xxv}
appears when the continuum level is high. We also detect a
constant, unresolved Fe~K\alpha line (EW~~80~eV)
with a center energy consistent with the K\alpha line from
Fe~{\sc i} to Fe~{\sc xvii} which is not redshifted,
originating at >2.1 light days from the X-ray source. We
present the results of a study of Fe~K\alpha lines in a
Chandra HETGS sample of type~I AGN, including a search
for Fe~{\sc xxv} and Fe~{\sc xxvi} K\alpha lines. These
lines provide a more promising tool for measuring black-hole
mass and spin than the low ionization-state fluorescent
Fe~K\alpha lines since the ambiguity of ionization state
is removed and there is no confusion with a non-disk line.